Cells of heart muscle are inter-connected by an extensive and varied system of linkages that is found all along the cell surfaces and that exists in addition to intercalated discs. The objective of this study is to characterize the linkage system by morphological identification of components and their combinations, specific staining properties, and susceptibility to specific enzymatic degradation. Functional studies will include ultra-structural changes of the linkage system with passive stretch or high activation of the muscle and with osmotically induced myocyte volume changes. In work subsequent to this grant, it is proposed to test directly the mechanical properties of heart muscle with selective uncoupling of intercalated discs or the other linkage system. Preliminary experiments showed that EGTA pretreated, hyperpermeable rat ventricular muscles gave Ca ion-ATP forces greater than those from the electrically excited living muscles, even though the intercalated discs had been abolished by the low Ca ion pretreatment and the myocytes in the 2 mm-long muscle were only 100-150 micron m in length. Electron microscopy has revealed that the myocytes are inter-connected by various linkages involving cell coats, collagen fibrils, 2-4 nm wide microthreads, and other elements of the extracellular matrix (preliminary results, Robinson & Winegrad, 1978). A linkage system, distributed along the cell surfaces of the population of cells and having variable compliances, acting in concert with the more rigid intercalated discs at the cell ends, might well translate cellular force production and shortening into precise, localized movements, and effect transmission of subtle components of forces among cells of the muscle during the cardiac cycle. It might also act as a shock absorber and provide a part of the mechanism for the passive storage of systolic energy that is subsequently released during diastolic recoil. Its versatility and high turnover rates could help explain the plasticity of the heart in response to chronic stress.